The present invention relates to a process for producing carrier catalysts containing gold or a mixture of gold and iron oxide for oxidizing carbon monoxide below about 50.degree. C., to carrier catalysts produced according to the process of the invention, and to the use of such catalysts to oxidize carbon monoxide.
Carbon monoxide (CO) is a gas which is used in many industrial processes, for instance in the carbonylation of hydrocarbons to produce aldehydes. Furthermore, it is produced in chemical processes, particularly as a product of incomplete combustion, for instance in furnaces or internal combustion engines. Furthermore, it may be present in the ambient air of industrial plants or in parts of plants in which CO is used in operation, and also in coal mines. Due to the high toxicity of this colorless, odorless gas, it is desirable and, in the case of a correspondingly high concentration of CO, also necessary, to reduce the CO content in the air, in particular the inhaled air, to as great an extent as possible. Catalyzed oxidation, for instance with atmospheric oxygen, is particularly well suited for this purpose. Thus, extensive attempts have been made for a long time to develop catalysts which can be used for oxidizing CO.
For instance, catalysts for oxidizing CO have been developed which develop their catalytic activity only at high temperatures, for instance at or above 300.degree. C., and are therefore particularly well suited for treating hot combustion gases containing CO. In this type of application, the development of the catalytic activity only at such high temperatures is not a disadvantage, since combustion gases generally occur at such high temperatures. Also, there is naturally no possibility of contamination of the catalysts due to condensing water vapor. However, such catalysts cannot be used to oxidize CO at temperatures below 50.degree. C.
For catalysts which are intended to catalyze the oxidation of CO at temperatures below about 50.degree. C., for instance in air (usually containing water vapor) and which for instance are intended to be used in rescue apparatus for miners or the like, it is of decisive importance first that they must be catalytically active in the range of about -10.degree. C. to about 50.degree. C. Advantageously, they should also remain active in the presence of moisture. Other desirable properties are the most economical utilization possible of the catalytically active constituents, for instance noble metals, which are generally very expensive, and also good handling ability.
Catalysts are known which can catalyze the oxidation of CO even at temperatures below 50.degree. C. Hopcalite, a mixture of manganese dioxide, copper oxide, and if desired other metal oxides, for instance cobalt oxide and silver oxide, should be mentioned in particular. However, hopcalite has the decisive disadvantage that it is very rapidly deactivated by water vapor, which is always present in air. To be able to use hopcalite for the oxidation of CO in moist air, for instance in rescue apparatus, it is necessary to combine it with a drying agent, which of course likewise has only a limited capacity for water collection.
Besides hopcalite, which consists entirely or predominantly of catalytically active base metals, the catalytic activity of noble metals with respect to the oxidation of carbon oxide at ambient temperature in moist air has also been investigated.
In a publication in Chemistry Letters 1987, pages 405 to 408, authors M. Haruta, T. Kobayashi, H. Sano and N. Yamada mention catalytically active full catalysts of gold and a transition metal oxide, in particular .alpha.-Fe.sub.2 O.sub.3, which can be used for the catalytic oxidation of CO even at ambient temperature in moist air. The preferred catalysts consist of 5 atom-% gold and 95 atom-% iron in the form of .alpha.-Fe.sub.2 O.sub.3, that is to say about 11.5% by weight gold and 88.5% by weight .alpha.-Fe.sub.2 O.sub.3. In order to be effective, these catalysts not only need to have this special composition, but they must also have been produced in a special way, namely by co-precipitation of the constituents. Carrier catalysts prove ineffective for CO oxidation at temperatures below about 50.degree. C. From this a person skilled in the art has to draw the conclusion that carrier catalysts are not suitable for this purpose.